This is a new application of an epidemiology study among five health maintenance organizations (HMOs) to evaluate the incidence of drug-induced liver injury that results in hospitalizations and to assess the association between exposure to specific drugs and liver outcomes not attributable to non-drug cause. The proposed study will be based on collaborative research infrastructure that has already been established at the HMO Research Network, which is funded as a Center for Education and Research on Therapeutics by the Agency for Healthcare Research and Quality. Five HMOs in different regions of the U.S. will participate. They are Fallon Community Health Plan in central Massachusetts, Harvard Pilgrim Health Care in Eastern Massachusetts, Kaiser Permanente Colorado in Denver area, Kaiser Permanente Northwest in Oregon, and Lovelace Clinic in New Mexico, with a combined health plan population of more than two million. Automated hospital discharge data from the HMOs will be used to identify all patients hospitalized with acute liver disease in 2001. Medical records of potential cases will be reviewed and the liver disease attributed to pre-defined causes. Cases not clearly attributed to a non-drug cause will be independently reviewed by two hepatologists to determine their severity and etiology. Cases of probably drug-induced acute liver injury, possibly drug-induced acute injury, idiopathic acute liver disease, and those whose liver events did not have enough information to evaluate causality will be described. Population-based incidence of these acute liver events will be calculated, and they will form the case series in a case-control analysis designed to evaluate the strength of association between drug exposure and these liver events. Two control series (hospital controls and health plan controls) will be identified. This proposed study will serve as a long term valuable data source for the study of drug-induced liver injury.